This invention relates in general to bonding of high temperature metal structures and, more specifically to a method and apparatus for bonding structures, such as honeycomb cores to face sheets, using differential pressure applied across a thin metal foil to assure uniform bonding without deformation of the structure.
Multi-component metal structures, such as honeycomb core panels and face sheets, have long been bonded together by coating one component, such as the face sheets, with a brazing material, pressing the components together, then heating the assembly to the brazing temperature. Often, the required pressure is applied by heavy weights or a pile of heavy pellets pressing down on the assembly, such as described, for example, by Rupert et al in U.S. Pat. No. 3,846,903. These methods tend to produce less than optimally uniform bonds and often are limited to flat panels. The pressure producing hardware is difficult to precisely apply, often deforms or damages the structures being bonded and is labor-intensive.
Others have surrounded the structure to be bonded with a thin metal capsule that allows a vacuum to be drawn on the assembly during heating to the bonding temperature, as described by Dewey et al in U.S. Pat. No. 3,173,813 and Henry in U.S. Pat. No. 3,091,846. These techniques, however, require a complex support apparatus with heat exchanger plates abutting the capsule and parts or do not support the capsule sufficiently to prevent distortion during heating. Thin structures, such as honeycomb cores formed from thin metal often are slightly bent or crushed during these bonding processes because of the inflexibility of the capsule walls. These methods seem to be suitable only for the production of flat structures.
Beuyukian et al. in U.S. Pat. No. 3,514,842 describe a method of brazing face sheets to honeycomb core in which the face sheets and core are adhesively bonded together, then enclosed between a pair of sheets that are peripherally bonded together to form an evacuable chamber. This is unlikely to produce a precisely flat or otherwise contoured panel, since there is no solid, dimensionally precise, support for the panel during heating to the brazing temperature.
A method of bonding face sheets to a honeycomb core in which the face sheets themselves form a vacuum chamber to hold them against the core during bonding is disclosed by Hamilton et al in U.S. Pat. No. 4,117,970. With normal, relatively thick, face sheets, deformation of thin walled core is likely if sufficient pressure is provided to superplastically deform the face sheets into tight contact with the core.
The prior art methods and apparatus in general have difficulty in producing curved or tapered panels. The vacuum capsules surrounding the structure being bonded as used in the prior art are thick walled and heavy, so that non-uniform pressure is applied to the structure being bonded. With thin walled structures, such as very light weight, high performance, titanium core panels the core is likely to be distorted by excess pressure in some areas while insufficient pressure in other areas will result in poor bonds. Thus, there is a continuing need for improvements in methods and apparatus for bonding metal structures, such as face sheets to honeycomb cores, that overcome these and other problems of the prior art techniques.